Re: [PATCH 0/9] TRACE_IRQFLAGS wreckage

[Marco Elver]

On Thu, Aug 20, 2020 at 03:59PM -0400, Steven Rostedt wrote:
> On Thu, 20 Aug 2020 19:20:46 +0200
> Marco Elver <elver@xxxxxxxxxx> wrote:
> 
> > On Thu, Aug 20, 2020 at 04:58PM +0200, peterz@xxxxxxxxxxxxx wrote:
> > > On Thu, Aug 20, 2020 at 10:36:43AM -0400, Steven Rostedt wrote:  
> > > > 
> > > > I tested this series on top of tip/master and triggered the below
> > > > warning when running the irqsoff tracer boot up test (config attached).
> > > > 
> > > > -- Steve
> > > > 
> > > >  Testing tracer irqsoff: 
> > > >  
> > > >  =============================
> > > >  WARNING: suspicious RCU usage
> > > >  5.9.0-rc1-test+ #92 Not tainted
> > > >  -----------------------------
> > > >  include/trace/events/lock.h:13 suspicious rcu_dereference_check() usage!  
> > ...
> > > 
> > > Shiny, I think that wants something like the below, but let me go frob
> > > my config and test it.
> > > 
> > > ---
> > > --- a/drivers/cpuidle/cpuidle.c
> > > +++ b/drivers/cpuidle/cpuidle.c  
> > ...
> > 
> > With that applied (manually, due to conflicts), I still get warnings for
> > certain call locations with KCSAN on (that is with my fix from the other
> > email):
> > 
> > | =============================
> > | WARNING: suspicious RCU usage
> > | 5.9.0-rc1+ #23 Tainted: G        W        
> > | -----------------------------
> > | include/trace/events/random.h:310 suspicious rcu_dereference_check() usage!
> > | 
> > | other info that might help us debug this:
> > | 
> > | 
> > | rcu_scheduler_active = 2, debug_locks = 0
> > | RCU used illegally from extended quiescent state!
> > | no locks held by swapper/1/0.
> > | 
> > | stack backtrace:
> > | CPU: 1 PID: 0 Comm: swapper/1 Tainted: G        W         5.9.0-rc1+ #23
> > | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> > | Call Trace:
> > |  __dump_stack lib/dump_stack.c:77 [inline]
> > |  dump_stack+0xf1/0x14d lib/dump_stack.c:118
> > |  trace_prandom_u32 include/trace/events/random.h:310 [inline]
> > |  prandom_u32+0x1ee/0x200 lib/random32.c:86
> > |  prandom_u32_max include/linux/prandom.h:46 [inline]
> > |  reset_kcsan_skip kernel/kcsan/core.c:277 [inline]
> > |  kcsan_setup_watchpoint+0x9b/0x600 kernel/kcsan/core.c:424
> > |  is_idle_task+0xd/0x20 include/linux/sched.h:1671 		<==== inline, but not noinstr
> > |  irqentry_enter+0x17/0x50 kernel/entry/common.c:293 		<==== noinstr function
> > 
> 
> What happens if you apply the below patch?
> 
> -- Steve
> 
> diff --git a/lib/random32.c b/lib/random32.c
> index 932345323af0..1c5607a411d4 100644
> --- a/lib/random32.c
> +++ b/lib/random32.c
> @@ -83,7 +83,7 @@ u32 prandom_u32(void)
>  	u32 res;
>  
>  	res = prandom_u32_state(state);
> -	trace_prandom_u32(res);
> +	trace_prandom_u32_rcuidle(res);
>  	put_cpu_var(net_rand_state);
>  
>  	return res;

Thank you, this resolves the problem. It also works if I remove my 2
other patches (for now, I think I still need the recursion-guard but
it's not urgent, will send that separately).

And my apologies, it seems 5.9-rc1 is already broken, which I should
have noticed. I sent a separate patch, which should be picked up into
5.9: https://lkml.kernel.org/r/20200821063043.1949509-1-elver@xxxxxxxxxx

With that fix + start_critical_timings-switcheroo, this series is:

Tested-by: Marco Elver <elver@xxxxxxxxxx>

Thanks,
-- Marco